In recent years, new microfabrication techniques have been developed to achieve more highly-integrated and higher-performance for semiconductor integrated circuits (hereinafter, referred to as “LSIs”). Chemical mechanical polishing (hereinafter, referred to as “CMP”) is one of such techniques, and is frequently used for, for example, flattening interlayer insulating films, forming metal plugs, and embedding wiring in LSI production process, especially in the step of forming multilayer wiring (see, for example, U.S. Pat. No. 4,944,836).
Further, in order to achieve higher-performance LSIs, an attempt has recently been made to use, as wiring materials, copper-based metals such as copper, metal films mainly made of copper, and copper alloys (hereinafter, simply referred to as “copper-based metals”). However, it is difficult to microfabricate such copper-based metals by dry-etching generally used for forming conventional aluminum alloy wiring. For this reason, for example, a so-called damascene process is mainly employed, in which a thin film made of copper-based metal is deposited on an insulating film having previously-formed trenches to fill the trenches with the copper-based metal and then the thin film made of copper-based metal except for the trenches is removed by CMP to form embedded wiring (see, for example, Japanese Patent Application Laid-open No. 2-278822).
In a general metal CMP method for polishing a wiring metal, a polishing pad is bonded to a circular polishing table, the surface of the polishing pad is wetted with a metal-polishing liquid, a substrate having a metal film formed on the surface thereof is pressed against the polishing pad so that the metal film comes into contact with the polishing pad, the polishing table is rotated while a predetermined pressure (hereinafter, referred to as “polishing load”) is applied from the back surface thereof to remove convexes of the metal film by mechanical friction between the polishing liquid and the convexes of the metal film.
A metal-polishing liquid for use in CMP generally contains an oxidizing agent and abrasive grains, and if necessary, further contains a metal-oxide-dissolving agent, a metal anticorrosive, etc. The basic mechanism of CMP is believed to be that the surface of a metal film formed on a substrate is first oxidized by oxidizing agent and then the thus obtained oxidized layer is scraped away by abrasive grains. The oxidized layer covering concaves of the metal film surface is less likely to come into contact with a polishing pad and therefore does not undergo abrasion by abrasive grains. Therefore, the convexes of the metal film are removed with progression of CMP so that the surface of the substrate is flattened (see, for example, Journal of Electrochemical Society, Vol. 138, No. 11, pp. 3460 to 3464 (1991)).
At first, a major problem of CMP was generation of deterioration in flatness caused by a phenomenon in which the central part of the surface of embedded metal wiring is recessed like a dish due to isotropic polishing (hereinafter, referred to as “dishing”) or a phenomenon in which an interlayer insulating film is polished together with a wiring metal so that a recess is formed in the interlayer insulating film (hereinafter, referred to as “erosion”). In order to overcome such deterioration in flatness, a polishing method using a metal-polishing liquid containing a metal-oxide-dissolving agent composed of aminoacetic acid (glycine) or amidosulfuric acid and a protective film-forming agent such as benzotriazole has been proposed (see, for example, Japanese Patent Application Laid-open No. 8-83780). However, such a polishing method, wherein deterioration in flatness can be overcome due to the ability of the protective film-forming agent, such as benzotriazole, to form a protective film, is not always suitable because a polishing rate is significantly reduced.
Further, in order to solve problems such as dishing and erosion, for example, Japanese Patent No. 3371775 discloses a polishing liquid for copper-based metal, which contains substantially no abrasive grains. Japanese Patent No. 3371775 also discloses a technique for forming embedded metal wiring by mechanically polishing the surface of a metal film with the use of a polishing liquid containing an oxidizing substance for oxidizing a metal film to be polished, an organic acid for making an oxide formed by oxidization using the oxidizing substance water-soluble, and water, and if necessary, an anticorrosive (a protective film-forming agent). More specifically, Japanese Patent No. 3371775 discloses a method for forming copper wiring with the use of, for example, a polishing liquid containing substantially no abrasive grains but containing hydrogen peroxide, citric acid, and benzotriazole. However, such a method involves a problem in that a polishing rate under normal polishing conditions is in the range of 80 to 150 nm/min and the saturation of a polishing rate occurs so that a polishing rate does not exceed 200 nm/min even when a high polishing load of 300 g/cm2 or higher is applied.
In order to solve such a problem, a polishing method has been proposed for forming embedded metal wiring by mechanically polishing the surface of a metal film with the use of a metal-polishing liquid containing substantially no abrasive grains but containing an oxidizing substance, phosphoric acid, an organic acid, a protective film-forming agent, and water (see Japanese Patent Application Laid-open No. 2002-50595). This method makes it possible to achieve an increased polishing rate (700 nm/min or higher) and to obtain a polished surface with dishing or erosion of about 50 nm or less. Polishing using such a polishing liquid as described above containing substantially no abrasive grains is performed mainly by friction with a polishing pad. Therefore, the polishing method using the polishing liquid is required to apply a high polishing load. For example, in the case of the above-described method, a polishing load of 220 g/cm2 is applied.
In order to improve a polishing rate and the flatness of a polished surface, metal-polishing liquids containing an additive expressed as a surfactant or a water-soluble polymer have been proposed. However, it has been reported that such an additive is effective at improving a polishing rate and at the flatness of a polished surface but makes friction increase during polishing (see, for example, Japanese Patent Application Laid-open No. 2004-6628).
On the other hand, in order to achieve higher-performance LSIs, introduction of copper wiring and switching of an interlayer insulating film. That is, a conventional silicon oxide film is coming to switching to an insulating film having a low dielectric constant (Low-k film).
The use of a Low-k film as an insulating film makes it possible to achieve a lower parasitic capacitance between devices or wiring lines as compared to a case where a conventional silicon oxide film is used as an insulating film. Examples of such a Low-k film include inorganic films such as SiOF films and Si—H-containing SiO2 films, organic-inorganic hybrid films such as carbon-containing SiO2 (SiOC) films and methyl group-containing SiO2 films, and organic polymer films such as Teflon®-based polymer films, polyimide-based polymer films, polyallyl ether-based polymer films, parylene-based polymer films, and wholly aromatic polymer films. For example, an organic-inorganic hybrid film made of methyl group-containing SiO2: HSG2209S-R7 (trade name), which is manufactured by Hitachi Chemical Co., Ltd., has a relative dielectric constant of 2.8. Further, as for the example of the organic polymer film, SiLK (trade name), which is manufactured by Dow Chemical, as a wholly aromatic polymer is being contemplated for a material capable of achieving a relative dielectric constant of 2.6 to 2.8.
Further, a Low-k film having a lower relative dielectric constant is under study. For example, porous materials obtained by forming micropores in the above-mentioned materials are thought to be promising materials to achieve a relative dielectric constant of 2.5 or less.
However, such a Low-k film having a relative dielectric constant of 3 or less has low mechanical strength and shows poor adhesion to metal films or to other insulating films. Therefore, in the case of using such a Low-k film as an insulating film, there is a problem that cohesive failure of the Low-k film itself occurs or interface delamination occurs between the Low-k film and a film other than the Low-k film when a copper-based metal layer and a barrier layer are polished by CMP in a damascene process. It can be considered that such a problem is caused by a high polishing load or a high frictional force developed between a wafer and a polishing pad during CMP.
Further, in order to improve a polishing rate and the flatness of a polished surface, metal-polishing liquids containing an additive expressed as a surfactant or a water-soluble polymer have been proposed. However, it has been reported that such an additive is effective at improving a polishing rate and the flatness of a polished surface but increases friction during polishing (see, for example, Japanese Patent Application Laid-open No. 2004-6628).
As has been described above, such conventional polishing liquids for CMP are difficult to simultaneously satisfy two or more of the following requirements: to achieve a high polishing rate; to achieve high flatness of a polished surface; and to exhibit excellent performance even in the case of polishing the surface to be polished, having a film with low mechanical strength such as a Low-k film, and therefore there is room for improvement.